KVM: x86: rdpmc emulation checks the counter incorrectly

The rdpmc emulation checks that the counter (ECX) is not higher than 2, without
taking into considerations bits 30:31 role (e.g., bit 30 marks whether the
counter is fixed). The fix uses the pmu information for checking the validity
of the pmu counter.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>